BestDirect (NCIX Sister) has the for 84.47, and I've seen them on sale at NCIX every once in a while for $70.
Buy Western Digital Caviar Black 640GB 7200RPM 32MB Dual Proc 3.5IN SATA Hard Drive OEM 5YR Mfg Warranty
More information as to your budget and what you needs are, would be great as per miggs78 above!